Liquid crystal display devices using nematic liquid crystal display elements, which have conventionally been widely used as numeric-value-segment-type display devices such as watches and portable calculators, have recently been also used in word processors, note-type personal computers, car-use liquid crystal televisions, and other apparatuses.
Generally, a liquid crystal display element has a light-transmitting substrate and electrode lines for turning on and off pixels and other components that are formed on the substrate. For example, in an active-matrix liquid crystal display device, active elements, such as thin-film transistors, are formed on the substrate together with the electrode lines as switching means for selectively driving pixel electrodes by which voltages are applied across the liquid crystal. Further, in liquid crystal display devices capable of color display, color filter layers having colors such as red, green and blue are provided on the substrate.
Liquid crystal display elements such as that mentioned above adopt a liquid crystal display mode that is suitably selected depending on twist angles of the liquid crystal: some of well-known modes are active-driving-type twisted nematic liquid crystal display mode (hereinafter, referred to as the TN mode) and the multiplex-driving-type super-twisted nematic liquid crystal display mode (hereinafter, referred to as the STN mode).
The TN mode displays images by orienting the nematic liquid crystal molecules to a 90°-twisted state so as to direct rays along the twisted directions. The STN mode utilizes the fact that the transmittance is allowed to change abruptly in the vicinity of the threshold value of the applied voltage across the liquid crystal by expanding the twist angle of the nematic liquid crystal molecules to not less than 90°.
The problem with the STN mode is that the background of the display screen sustains a peculiar color due to interference between colors because of the use of the birefringence effect of liquid crystal. In order to solve this problem and to provide a proper black-and-white display in the STN mode, the application of an optical-retardation compensation plate is considered to be effective. Display modes using the optical-retardation compensation plate are mainly classified into two modes, that is, the double layered super-twisted nematic optical-retardation compensation mode (hereinafter, referred to as the DSTN mode) and the film-type optical-retardation compensation mode (hereinafter, referred to as the film-addition mode) wherein a film having optical anisotropy is provided.
The DSTN mode uses a two-layered construction that has display-use liquid crystal cells and liquid crystal cells which are oriented with a twist angle in a direction reversed to that of the display-use liquid crystal cells. The film-addition mode uses a construction wherein films having optical anisotropy are placed. Here, the film-addition mode has been considered to be more prospective on the standpoint of light weight and low costs. Since the application of such an optical-retardation compensation mode makes it possible to improve the black-and-white display characteristics, color STN liquid crystal display devices, which enable color display by installing color-filter layers in STN-mode display devices, have been achieved.
The TN modes are, on the other hand, classified into the normally black mode and the normally white mode. In the normally black mode, a pair of polarization plates are placed with their polarizing directions in parallel with each other, and a black display is provided in a state where no on-voltage is applied across the liquid crystal layer (off-state). In the normally white mode, a pair of polarization plates are placed with their polarizing directions orthogonal to each other, and a white display is provided in the off-state. Here, the normally white mode is considered to be more prospective from the standpoints of display contrast, color reproducibility, viewing-angle dependence, etc.
However, in the TN-mode liquid crystal display devices, the liquid crystal molecules have a refractive index anisotropy Δn, and are oriented so as to incline to the two substrates that are disposed opposite to each other. For these reasons, the viewing-angle dependence increases: i.e., the contrast of displayed images varies depending upon the direction and angle of the viewer.
FIG. 17 schematically shows the cross-sectional construction of a TN liquid crystal display element 41. As a result of application of a voltage for half-tone display, liquid crystal molecules 42 shown in FIG. 17 slants upward slightly. In such a liquid crystal display element 41, a linearly polarized ray 45 passing through the surfaces of a pair of substrates 43 and 44 along the normals thereto, and linearly polarized rays 46 and 47 passing through those surfaces not along the normals thereto cross the liquid crystal molecules 42 at different angles. Besides, the liquid crystal molecules 42 have a refractive index anisotropy an. Therefore, the linearly polarized rays 45, 46 and 47, upon passing through the liquid crystal molecules 42 in different directions, produce ordinary and extraordinary rays. The linearly polarized rays 45, 46 and 47 are converted to elliptically polarized rays according to the phase difference between the ordinary and extraordinary rays, which cause the viewing-angle dependence.
In addition, in an actual liquid crystal layer, the liquid crystal molecules 42 show different tilt angles in the vicinity of the midpoint between the substrates 43 and 44 and in the vicinities of the substrates 43 and 44. The liquid crystal molecules 42 near the substrate 43 and those near the substrate 44 are twisted by 90° about the normal.
For those reasons described so far, the linearly polarized rays 45, 46 and 47 passing through the liquid crystal layer are affected by the birefringence effect in various ways depending upon, for example, the direction and the angle thereof, resulting in complex viewing-angle dependence.
Such viewing-angle dependence can be observed, as examples, in the following situations. If the viewing angle increases from the normal to the display screen in the standard viewing direction, i.e. downward, and exceeds a certain angle, the displayed image has a distinct color (hereinafter, referred to as the coloration phenomenon), or is reversed in black and white (hereinafter, referred to as the reversion phenomenon). If the viewing angle increases from the normal in the opposite viewing direction, i.e. upward, the contrast decreases abruptly.
The aforementioned liquid crystal display device has another problem that the effectual range of viewing angle narrows with a larger display screen. When a large liquid crystal display device is viewed from a short distance in the front thereof, the same color may appear different in the uppermost and lowermost parts of the large screen due to the effect of the viewing-angle dependence. This is caused by a wider range of viewing angle required to encompass the whole screen surface, which is equivalent to a viewing direction which is increasingly far off-center.
To restrain the viewing-angle dependence, Japanese Laid-Open Patent Applications No. 55-600/1980 (Tokukaisho 55-600) and No. 56-97318/1981 (Tokukaisho 56-97318) suggest that a phase difference plate (phase difference film) be inserted as an optical element having optical anisotropy between the liquid crystal display element and one of the polarization plates. According to the method, the elliptically polarized ray converted from a linearly polarized ray by passage through liquid crystal molecules having refractive index anisotropy is passed through the phase difference plate(s) disposed on the side(s) of the liquid crystal layer having refractive index anisotropy. Hence, the phase difference between the ordinary and extraordinary rays are compensated for for all viewing angles, and the elliptically polarized ray is converted back to the linearly polarized ray, which enables the restraint of the viewing-angle dependence.
Japanese Laid-Open Patent Application No. 5-313159/1993 (Tokukaihei 5-313159), as an example, discloses a phase difference plate of the above kind represented by a refractive index ellipsoid with one of the principal refractive indices parallel to the normal to the surfaces of the phase difference plate. Nevertheless, this phase difference plate still cannot satisfactorily restrain the reversion phenomenon that occurs when the viewing angle increases in the standard viewing direction.
To solve the problem, Japanese Laid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116, corresponding to U.S. Pat. No. 5,506,706) suggests the use of a phase difference plate represented by a refractive index ellipsoid with the principal refractive indices inclining to the normal to the surfaces of the phase difference plate. This method adopts two kinds of phase difference plates as follows.
One of the phase difference plates can be represented by such a refractive index ellipsoid that the smallest of the three principal refractive indices is parallel to the surfaces, one of the larger principal refractive indices inclines to the surfaces of the phase difference plate by an angle θ, the remaining principal refractive index inclines to the normal to the phase difference plate by the same angle θ, and the angle θ satisfies 20°≦θ≦70°.
The other phase difference plate can be represented, in terms of the refractive index anisotropy thereof, by a refractive index ellipsoid inclining to the surfaces thereof. To be more specific, the phase difference plate is such that the three principal refractive indices na, nb, and nc of the refractive index ellipsoid satisfy na<nc<nb, and that the principal refractive index nb and one of the other principal refractive indices na or nc which lies in the surface plane incline clockwise or counterclockwise about the remaining principal refractive index nc or na.
As the former phase difference plate, a uniaxial or biaxial phase difference plate can be used. For the latter one, two phase difference plates, instead of one, can be used in such a combination that the two principal refractive indices nb form an angle of 90°.
A liquid crystal display device, incorporating at least one such phase difference plate between the liquid crystal display element and the polarization plate exhibits some restraint in the contrast variations, coloration phenomenon, and reversion phenomenon caused by the viewing-angle dependence of the display screen.
However, with today's increasingly large demand on a wider effectual range of viewing angle and superb display quality, a better restraint in the viewing-angle dependence is crucial. In this context, the phase difference plate disclosed in Japanese Laid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116) above does not provide satisfactory solutions and needs to be improved.